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The aim of the paper is to explain the reason why the creep rupture on welded joints at high temperatures is often observed in the heat affected zone (HAZ), especially in the so called intercritical zone (IC-HAZ). The creep rupture strength (CRS) of cross-weld joints is usually about 20 to 30% lower than that of the base material in the case of the creep resistant modified chromium steel. Precipitation strengthening (PS) depends on the interparticle spacing (IPS) of secondary phases — M23C6 and nanoparticles of MX. With increasing IPS of secondary phases the proof stress and creep rupture strength decreases, while creep rate increases. To estimate the IPS in HAZ the modelling technique was used for estimation of the microstructure and properties. The microstructure of HAZ modelled regions was analysed by transmission electron microscopy. The Smitweld machine was used to reproduce the thermal cycles in HAZ. Real creep tests were carried out on specimens with HAZ modelled regions and also on the P91 real welded joints. It was found that interparticle spacing of secondary phases in the HAZ is much higher than that in the base material. Therefore the proof stress and hardness as well as creep rupture strength of HAZ is much lower than that of the base material. It was observed that CRS in the HAZ is about 20% lower than that of the base material.

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